The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines combust an air and fuel mixture to generate drive torque. The combustion process generates exhaust that is exhausted from the engine to atmosphere. The exhaust contains nitrogen oxides (NOx), carbon dioxide (CO2), carbon monoxide (CO) and particulates. NOx is a term used to describe exhaust gases that consist primarily of nitrogen oxide (NO) and nitrogen dioxide (NO2). An exhaust after-treatment system treats the exhaust to reduce emissions prior to being released to atmosphere. In an exemplary exhaust after-treatment system, a dosing system injects a dosing agent (e.g., urea) into the exhaust upstream of a selective catalytic reduction (SCR) catalyst. The exhaust and dosing agent mixture reacts over the SCR catalyst to reduce the NOx levels released to atmosphere.
The dosing agent reacts with NOx on the SCR catalyst to accomplish the NOx reduction. More specifically, the dosing agent breaks down to form ammonia (NH3), which is the reductant utilized to react with the NOx. The following exemplary, chemical relationships describe the NOx reduction:4NO+4NH3+O2→4N2+6H2O4NH3+2NO+2NO2→4N2+6H2O3NO2+4NH3→3.5N2+6H20
To perform the above-described NOx reduction, the SCR catalyst stores NH3 therein. For an SCR catalyst to perform effectively, the NH3 storage level must be maintained at an adequate level. More specifically, the NOx reduction or conversion efficiency is dependent upon the NH3 storage level. In order to maintain high conversion efficiency under various operating conditions, the NH3 storage must be maintained. However, as the temperature of the SCR catalyst increases, the NH3 level must be reduced to avoid NH3 slip (i.e., excess NH3 being released from the SCR catalyst), which can reduce the conversion efficiency of the catalyst.